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Noisy Filters Snare Tiny Particles

It seems like common sense: a filter's mesh must be finer than the particles it's meant to catch. But a group of engineers has now shown that--with the help of a bit of noise--filters with a relatively wide mesh can trap very tiny particles. Engineers say that the technique may soon be used in fields ranging from biotechnology to waste management.

Filters are designed to snare all kinds of particles floating in liquids. That task is relatively simple for large objects like coffee grounds and spaghetti strands, but to snare microscopic contaminants like bacteria and lead particles, designers often must use superfine membranes. These membranes clog and tear easily, however, so engineers have put a great deal of effort into improving them.

Why not use sound waves to coax filters with big pores into catching tiny particles, asked chemical engineer Donald Feke and his colleagues at Case Western Reserve University in Cleveland, Ohio. Since the 1930s, scientists have known that high-frequency sound waves can push particles through liquid, so the right noise might shove them against the sides of a filter as they drift around randomly, allowing the filter to catch them. To test the idea, the researchers placed a rectangular mesh with millimeter-sized holes inside a small tank of water where a light current kept the liquid circling. Next, they placed latex particles a few micrometers wide on one side of the tank and applied an acoustic field using a vibrating ceramic plate. At the Society of Rheology's annual meeting last month, they reported that the filter trapped a respectable 90% of the particles in a single pass.

The technique is "fascinating" says Terence Coakley, a biophysicist at Cardiff University in the United Kingdom. He says that if further tests show that the technology also works on large scales, it could be used to separate cells from solution without damaging them and to help treat sewage.